Landforms of British Columbia 1976 - Department of Geography

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tinctive mesa-like or step-like form. For example, erosion of an extensive flat-lying bed of cliff-forming sandstone or conglomerate may result in an essentially ‘flat upland surface surrounded by escarpments related in height to the thickness of the resistant bed (see Plate XXXVIII) . Alternatively, areas underlain by flat-lying shales, which are more readily eroded, commonly display a uniformly low relief. Plateaus, plains, and areas of low relief are commonly associated with areas of flat- lying sedimentary rocks. Figure 3. Diagram showing the main areas of flat-lying or gently dipping sedimentary rocks. Areas of flat-lying or only very gently dipping sedimentary rocks are shown on Figure 3. By far the largest area is in the northeastern corner of the Province, where such rocks underlie the country east of the foothills of the Rocky Mountains. Other areas are the Spatsizi Plateau at the head of the Stikine River, the north- east tip of Graham Island, and the delta of the Fraser River downstream from Chilliwack. (3) Molten lava erupted from a single volcanic vent or from a number of fissures may spread over considerable areas. When erupted, the lava is fluid and mobile, and extensive individual flows may be essentially flat or have a slope that represents the original flow gradient. Lava flowing out over the land surface fills in the lower areas and eventually may accumulate to considerable thicknesses. In this way a flat surface may be built up on an older surface of some relief, and a plateau of constructional origin is formed. Consolidated lava is resistant to erosion, and flat-lying flows resemble flat- lying resistant sedimentary beds inasmuch as they produce a plateau topography. 17
As Iava cools and consolidates, it characteristically deveIops a vertica1 cohmmar jointing, the columns being at right angles to the cooling surface. This verticaI jointing is so we11 developed that dissection of a succession of Iava flows often produces a step-like profile, the treads being controlled by the individual horizontal flows, and the risers by the vertical joints in them. Vertical cliffs, each the height of a flow’s thickness, or escarpments hundreds of feet high, are characteristic of areas of dissected lava. Figure 4. Diagram showing the main areas of flat-lying or gently dipping lava flows. Figure 4 shows the area1 distribution of flat-lying or gently dipping lava flows. They occur 1argeIy within the southern and central interior of the Province, and extend over considerable areas between Penticton in the south and Houston in the north. A very large area west of the Fraser River in the drainage basin of the Chilcotin, West Road, and Nechako Rivers is underlain by flat-lying lavas (see Plate XXVI.4). Comparable rocks underlie areas of considerable size centring around Mount Edziza and Meszah Peak (see Plate XVIIB) and in the Kawdy Plateau west of Tuya Lake. Gently dipping lavas underlie a somewhat smaller area in the northwest part of Graham IsIand (see Plate 11~). (4) The tilting of sedimentary rocks allows the differences in resistance between beds to control the topography. Streams erode the softer beds faster, and consequently peaks and ridges are underlain by more resistant beds than are the valleys and depressions. Folding and faulting of sedimentary rocks develop structures in which tilted beds may extend for many miles, and erosion of this sort of terrain results in topography of a very strongly linear character. The 18
Page 1 and 2: Bulletin 48 Landforms of British Co
Page 3 and 4: The technical references are believ
Page 5 and 6: PAGE Chapter II.-Physiographic Subd
Page 7 and 8: FIQ. ILLUSTRATIONS 1. Landform map
Page 9 and 10: PLATE PAOE XVIII. (33) Stikine Plat
Page 11 and 12: PLATE XXXVIIB. (70) Rocky Mountain
Page 13 and 14: traveller. If he SO wishes, his enj
Page 15: pendants which, because of their se
Page 19 and 20: are lithologic differences from pla
Page 21 and 22: mentary rocks are folded and faulte
Page 23 and 24: Field work by members of the Geolog
Page 25 and 26: BIBLIOGRAPHY Alden, W. C.: “ Phys
Page 27 and 28: The western boundary of the Eastern
Page 29 and 30: INSULAR MOUNTAINS The Insular Mount
Page 31 and 32: H. C., “ Geology and Mineral Depo
Page 33 and 34: The extreme northeastern tip of Gra
Page 35 and 36: the strait. It is suggested that th
Page 37 and 38: The lowland consists of many low, w
Page 39 and 40: anges farther west. The ranges sout
Page 41 and 42: Pacific Ranges The Pa&c Ranges (see
Page 43 and 44: Columbia. The mountains along the 4
Page 45 and 46: ones, and widening and deepening th
Page 47 and 48: south end of Teslin Lake, which sep
Page 49 and 50: Between the Taku &nd Sheslay Rivers
Page 51 and 52: The tuyas consist of nearly horizon
Page 53 and 54: sloping areas of upland remain undi
Page 55 and 56: mostly in the Babine Range. The roc
Page 57 and 58: to Eutsuk Lake on the southern side
Page 59 and 60: I These valleys are at a fairly hig
Page 61 and 62: Detailed studies made around severa
Page 63 and 64: The area is drained by the Finlay,
Page 65 and 66: Plateau and the Nechako Plateau, an
Page 67 and 68:
iver in pre-Pl&ocene time or whethe
Page 69 and 70:
dissected upland surface of the pla
Page 71 and 72:
Thompson and North Thompson River v
Page 73 and 74:
Adams, and Shuswap Rivers and their
Page 75 and 76:
highlands, but the main effects of
Page 77 and 78:
eda Rivers on the west and the Colu
Page 79 and 80:
The amount of relief is governed by
Page 81 and 82:
“ Striae, roches moutonn&s, and l
Page 83 and 84:
The boundary between the Rocky Moun
Page 85 and 86:
[References: Price, R. A., “ Fern
Page 87 and 88:
8,820 feet, and the lowest point in
Page 89 and 90:
margin of the Rocky Mountains% it s
Page 91 and 92:
Devonian limestones that are thrust
Page 93 and 94:
locally, as a rule only on the nort
Page 95 and 96:
Dept. of Mines and Petroleum Remmx?
Page 97 and 98:
CHAPTER III.-SPECIAL ASPECTS OF THE
Page 99 and 100:
movement, and regional studies of t
Page 101 and 102:
Both the Atlas of British Columbia
Page 103 and 104:
ase of the Sumas till in the Fraser
Page 105 and 106:
British Columbia includes the drain
Page 107 and 108:
The Parsnip and Finlay Rivers were
Page 109 and 110:
upper Fraser River is the result of
Page 111 and 112:
Its pattern of flow within the moun
Page 113 and 114:
materials. Along the west coast of
Page 115 and 116:
, Pleistocene probably has been bet
Page 117 and 118:
A small volcanic cone with associat
Page 119 and 120:
‘I , titularly for those interest
Page 121 and 122:
The Pinchi fault and its northweste
Page 123 and 124:
component of the discordant pattern
Page 125 and 126:
APPENDIX B.-GLOSSARY * Aggrade-to b
Page 127 and 128:
Lineation-in rock the parallel orie
Page 129 and 130:
INDEX The place names listed are th
Page 131:
85 126 :: :: 116
Page 134 and 135:
Quam Quartz ~ountam Queen Bess, Mou
Page 136:
138
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Landforms of British Columbia 1976 - Department of Geography

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